1. Field of the Invention
This invention relates to solid electrolytic capacitors using a conductive polymer layer as a solid electrolyte and methods for manufacturing the same.
2. Description of Related Arts
With the recent trend towards smaller and lighter electronic devices, small-sized and high-capacitance capacitors for high frequency are being desired. Among the capacitors heretofore proposed as small-sized and high-capacitance capacitors are solid electrolytic capacitors in which the surface of an anode formed of a sintered body of a valve metal, such as tantalum, niobium, titanium, or aluminum, is anodized to form a dielectric layer and a solid electrolyte layer is provided on the dielectric layer. When in such a solid electrolytic capacitor a conductive polymer is used as the solid electrolyte layer, the equivalent series resistance (ESR) can be reduced.
Conceivable methods for increasing the capacitance of a solid electrolytic capacitor are, for example, to use a high-relative permittivity material as an anode material, to increase the surface area of the anode, and to reduce the thickness of the dielectric layer.
To increase the surface area of the anode, it is conceivable to use a sintered body of a valve metal powder with a small particle diameter or use a valve metal subjected to a treatment for increasing the surface area, such as an etched sheet of valve metal foil. However, it is difficult to completely coat the entire surface of the dielectric layer with a cathode layer, and the portions of the capacitor in which the dielectric layer is not coated with the cathode layer do not develop a capacitance. In addition, even once the dielectric layer has been entirely coated with the cathode layer, the resultant capacitor may have a problem with long-term reliability, such as gradual reduction in capacitance due to exposure to high temperatures or voltage application.
The method of reducing the thickness of the dielectric layer has a problem in that if the thickness of the dielectric layer is excessively reduced, leakage current flows between the anode and the cathode layer in incomplete portions, defective portions, pinholes and the like of the dielectric layer.
JP-A-H02-74021 describes that the adhesion between the dielectric layer and the conductive polymer layer can be improved by subjecting the surface of the dielectric layer to a surface treatment using a silane coupling agent.
JP-A-H11-329900 proposes a method in which in preparing a conductive polymer compound, a silane coupling agent is previously added into a chemically polymerizable liquid for forming the conductive polymer. The literature describes that with the use of this method, the adhesion between the dielectric layer and the conductive polymer layer can be improved, so that a low-ESR solid electrolytic capacitor can be provided.
JP-A-2010-103489 proposes to treat the surface of the dielectric layer with a phosphonic acid surface treatment agent. The literature describes that through this treatment, the capacitance can be increased, the ESR can be reduced, and the leakage current can be reduced.